WASHINGTON -- The U.S. has set up a small Marine artillery outpost in northern Iraq to protect a nearby Iraqi military base, expanding the number and the combat exposure of American troops in the country as Iraqi security forces plan and prepare for a counteroffensive against the Islamic State in Mosul.

It is the first such base established by the U.S. since it returned forces to Iraq in 2014 in response to the Islamic State's takeover of Mosul and other areas of northern and western Iraq, Col. Steve Warren, spokesman for the U.S. military headquarters in Baghdad, told reporters at the Pentagon Monday. He said it should not be considered a combat outpost because it is located behind the front lines and is not initiating combat with the militants.

"Their primary mission is to protect, obviously, Americans," Warren said, referring to the U.S. advisers at an Iraqi base near Makhmour. He declined to reveal the number of U.S. advisers but said it is fewer than 100. He said the Marine outpost, dubbed Fire Base Bell, is "several hundred meters" from the Iraqi base.

It did not take Islamic State fighters long to notice the additional Marines. On Saturday, they fired two rockets at the site, killing one Marine.

The addition of an artillery unit of close to 200 Marines reflects thorny decisions ahead for the Obama administration, which is intent on avoiding the appearance of an expanding U.S. combat mission in Iraq more than four years after President Barack Obama pulled U.S. troops out of the country. That political reluctance is further fueled by pockets of vocal opposition in Iraq to any increase in the number of American forces there.

The sensitivity over force numbers has added a layer of confusion to the troop totals. The White House has capped the number at about 3,870, but that total doesn't include as many as 1,000 forces who are exempt because of the military's personnel accounting system.

For example, although Fire Base Bell is surrounded by wire and fortified with four 155mm howitzers, the Marines there are not included in the troop totals because their deployment is considered a temporary assignment. Officials have said the Marines, who are members of the 26th Marine Expeditionary Unit, are likely to return to their ship at some point but would probably be replaced by rotations of other troops on similar short-term deployments.

Other troops not included in the official total are those working at the embassy, those assisting with foreign military sales to Iraq and those involved in other defense support missions. As an example, Warren is not included in the total because his assignment is considered short-term.

The Marine fire base has also proven to be vulnerable.

A member of the unit, Staff Sgt. Louis F. Cardin of Temecula, California, was killed by rocket fire Saturday. Peter Cook, the Pentagon press secretary, said several other Marines were wounded in the attack. Warren said Monday that some of the wounded were quickly returned to duty and an undisclosed number of others were evacuated to a U.S. military hospital in Germany.

At a news conference in Cuba on Monday, Obama expressed his condolences.

"It's a reminder that even as we embark on this historic visit, there are U.S. armed service members who are sacrificing each and every day on behalf of our freedom and our safety, so I'm grateful to them," the president said.

On Monday, a small number of Islamic State fighters got close enough to the Marines' outpost to target them with small arms, Warren said. He said no U.S. personnel were wounded in that attack, but two Islamic State fighters were killed.

In response to questions, Warren said the Marines' mission is defensive and is considered part of the advise-and-assist mission the U.S. military is undertaking on the Iraqi military base located near Makhmour, between the cities of Mosul and Kirkuk. The Iraqis are preparing ground combat forces for an eventual mission to recapture Mosul. Warren said the Marines began arriving at the fire base about two weeks ago.

U.S. military officials said there is currently no plan for the Marine artillery unit to be used in any offensive mission to retake Mosul, but they wouldn't rule it out. The officials were not authorized to discuss the matter publicly so spoke on condition of anonymity.

The existence of the Marine base was announced Sunday, one day after Cardin's death. Warren said the military delayed the announcement to protect the security of the troops as they moved into place. And he said that, also for security reasons, he could not disclose the exact number of Marines at the fire base but that it was a "company," which normally numbers between 100 and 200 troops.